[BUG][JAVA][FEIGN] Code generation outputs non-compilable code.

Description

When building with Feign, the generated code can not be compiled because of the missing OAuth.class on the classpath.

When in the JavaClientCodegen.java the classes OauthPasswordGrant.java and OauthClientCredentialsGrant.java get added we need also the OAuth-class to be added because the 2 mentioned classes extend this abstract-class.

This can be seen in the mustache files OauthPasswordGrant.mustache and OauthClientCredentialsGrant.mustache

openapi-generator version

the version I tested is 5.1.1

Generation Details

This codegeneration is run with following gradle-task:

openApiGenerate {
    generatorName = "java"
    inputSpec = "$buildDir/openapi.json"
    outputDir = "$buildDir/generated-client"
    apiPackage = "our.package.api"
    invokerPackage = "our.package.invoker"
    modelPackage = "our.package.model"
    configOptions = [
            dateLibrary       : "java8",
            interfaceOnly     : "true",
            debugModels       : "true",
            java8             : "true",
            ensureUniqueParams: "false",
            library           : "feign"
    ]
    globalProperties = [
            modelDocs: "false"
    ]
    skipValidateSpec = true
    logToStderr = true
    generateAliasAsModel = false
    enablePostProcessFile = false
}

execution of a build with the generated gradle.build results in a compilation error that OAuth.class is not found.

Suggest a fix

add following line in the javaClientCodegen in the if-clause of Feign (if (FEIGN.equals(getLibrary()))):

supportingFiles.add(new SupportingFile("auth/OAuth.mustache", authFolder, "OAuth.java"));

Start in JavaClientCodegen.java and inspect the Feign branch where supporting files are registered. Compare OauthPasswordGrant.mustache and OauthClientCredentialsGrant.mustache with the available auth templates, then generate a Feign client using the reported Gradle configuration. Done means the generated OAuth.java is present and the generated project compiles without the missing OAuth.class error.